The present invention relates generally to radar systems, and more particularly, to a system and method for near range microwave detection for use with frequency-modulation continuous-wave (FMCW) and stepped frequency radar systems.
Several partial solutions exist for the well-known near-range FMCW detection problem. These include physically separating the transmit and receive antennas to improve isolation, cancellation of leakage by analog hardware methods, minimizing internal reflections within the system, and increasing FM ramp bandwidth. Having physically distinct transmit and receive antennas increases hardware complexity, size and cost, as does hardware cancellation. Hardware cancellation and minimization of internal reflections are unattractive for mass-production. These techniques are available but can not be implemented in an automotive product:because of cost, size and the difficulties of mass production. Increasing FM ramp bandwidth is often precluded by FCC regulations or may require operation in an alternative frequency band where microwave components are less producible.
Consequently, it is an objective of the present invention to provide for a system and method based on waveform design and digital signal processing that provides for detection of near-range targets using a frequency-modulation continuous-wave (FMCW) or stepped frequency radar and that overcomes the above-mentioned problems and limitations.
In order to meet the above and other objectives, the present invention employs a cancellation procedure embodied in a digital signal processor of a radar system that is used to detect near-range targets. It was previously assumed that it was not possible to detect targets in the xe2x80x9cfirst range binxe2x80x9d formed by processing the return of an FMCW waveform. The present invention allows for detection of targets within this regime, thus permitting the use of a narrower bandwidth system for a given near obstacle range.
The present system and method uses a frequency-modulation continuous-wave (FMCW) or stepped frequency waveform to detect targets that would normally be obscured by transmitter leakage and internal reflections. The cancellation procedure is implemented by transmitting one or more reference ramp signals and then subtracting the coherent average of the transmitted reference ramp signals from each signal in a group of succeeding transmitted and received ramp signals. The resulting group of FM ramp return signals is then noncoherently integrated to achieve more stable target detection statistics and to provide for detection of nearby targets.
In its simplest form, the present invention is a method of detecting near range targets for use with frequency modulation continuous wave (FMCW) and stepped frequency waveforms. The method comprises the steps of: transmitting reference ramp signals; receiving reference ramp return signals; determining a coherent average of the received reference ramp return signals; subtracting a coherent average of the transmitted reference ramp signals from each signal in a group of succeeding reference ramp return signals to produce cancelled ramp signals; noncoherently integrating the resulting cancelled ramp signals to provide for detection of nearby targets; and displaying target data for a target whose position is less than a predetermined range.
More particularly, one detailed method of providing near range detection of targets for use with frequency-modulation continuous-wave (FMCW) and stepped frequency radar systems in accordance with the present invention comprises the following steps. Generating a predetermined number of reference ramp signals. Coherently averaging target return signals corresponding to transmitted reference ramp signals in the time domain to produce a reference average signal. Storing the resulting reference average signal. Transmitting an additional number of ramp signals. Subtracting the reference average signal from each ramp signal as it is received. Generating a Fourier transform signal for each ramp signal. Magnitude detecting the Fourier transformed signal. Noncoherently integrating the magnitude detected signal. Thresholding the integrated signal. Centroiding the thresholded signal, wherein centroid position is indicative of the range to a target. Displaying target data for a target whose range is less than a predetermined cutoff range.
In its simplest embodiment, the present invention is a radar system that provides for near range detection of targets for use with frequency modulation continuous wave (FMCW) and stepped frequency waveforms. The system comprises transceiver for transmitting reference ramp signals and receiving reference ramp return signals. A signal processing is coupled to the transceiver and provides for (a) determining a coherent average of the received reference ramp return signals; (b) subtracting a coherent average of the transmitted reference ramp signals from each signal in a group of succeeding reference ramp return signals; and (c) noncoherently integrating the resulting group of reference ramp return signals to provide for detection of nearby targets. A display is coupled to the signal processor for displaying detected targets.
A detailed embodiment of a radar system that provides for near range detection of targets in accordance with the present invention comprises an antenna/transceiver assembly, analog to digital conversion circuitry, and a digital signal processor. The antenna/transceiver assembly is conventional and comprises a transmit antenna, a receive antenna, a voltage controlled oscillator, a power divider having an input coupled between an output of the voltage controlled oscillator and the transmit antenna, a low noise amplifier coupled to an output of the receive antenna, and a homodyne mixer coupled to an output of the low noise amplifier and to a second output of the power divider. The analog to digital conversion circuitry comprises a D/A converter having an input coupled to the voltage controlled oscillator, and an A/D converter having an input coupled an output of the mixer.
The digital signal processor implements the improvement provided by the present invention and comprises waveform control circuitry that is coupled to the D/A converter and which is adapted to generate the ramp signals. A coherent average reference signal generator and a cancellation algorithm operate on data received from the A/D converter. The coherent average reference signal generator is adapted to generate and store the reference average signal from the received target return signals. The coherent average reference signal generator data is coupled to the cancellation circuit and is adapted to couple the reference average signal thereto for subtraction from each received ramp signal. A zero-padded fast Fourier transform (FFT) filter is coupled to the cancellation circuit that Fourier transforms each ramp signal. The Fourier transformed ramp signal is then serially processed by a magnitude detection unit, a noncoherent integrator, a thresholding unit, a centroiding unit, and a range cut-off unit. A display is coupled to an output of the range cut-off unit for displaying target data whose position is less than a predetermined cutoff range.